Accommodating Intraocular Lenses were developed in the early 1900's and have been sold in Europe for the last ten years and later in the U.S. They function by means of forward movement of the optic upon constriction of the ciliary muscle which increases the pressure in the posterior part of the eye with a simultaneous decrease in pressure in the front part of the eye pressure. The reverse pressure changes take place upon relaxation of the ciliary muscle, which results in the backwards movement of the lens for distance vision. The forward movement of the lens optic enables the patient implanted with the lens to automatically change their vision from distance to see at intermediate and near.
The first intraocular lenses developed by Harold Ridley were implanted in patients at St. Thomas' Hospital in London in 1949. The lenses were made of “perspex” (polymethylmethacrylate) and were large, heavy, rigid and biconvex. The complication rate was high since the specific gravity of PMMA was heavier than that of the aqueous that they were surrounded by, and they tended to de-center, or dislocate. This was just one of the complications of this first design. These lenses were placed in the eye behind the iris and in front of the posterior capsule of the human lens after the cataract had been removed, leaving behind the posterior capsule.
Subsequent lens designs by Epstein, Binkhorst and Worst attached the lens to the iris. This, along with anterior chamber lenses reduced the complication rate but it was not until Shearing developed a lens to be implanted into the empty capsular bag, putting the lens optic, once again back where it belonged, behind the iris, that there was a significant reduction in complications. This posterior chamber lens design was the first to have open loops attached to the optic, which helped to center and fixate the lens within the capsular bag by capturing the loops within the bag by means of fibrosis of the anterior capsule remnants to the posterior capsule over the flexible loops. These loops were called haptics.
Many iterations of this design followed. All the optics of these lenses were made of rigid PMMA. In the 1970's Mazzocco developed a single piece posterior chamber lens made from silicone that could be folded. This lens was molded as one piece, had no loops, and had what are now called “plate haptics”, which replaced the loops. At the time phacoemulsification had been developed and the cataract could be extracted through a 3-4 mm incision. However, since the earlier lenses had been made of rigid PMMA and had an optic diameter of 5-6 mm, the wound had to be enlarged to allow the lens to be inserted into the eye and had to be sutured. The advent of Mazzocco's foldable lens changed all this. It enabled foldable lenses to be inserted into the eye, folded, through a 3-4 mm tunnel incision, that now needs no suturing.
The word “haptic” has been used to describe an attachment to intraocular lenses. The original intraocular lens consisted of a single optic. These lenses frequently de-centered. It was discovered that there was a need to center and fixate the lens optic in the vertical meridian of the eye. The first attachments to the optic were called “haptics”. They consisted of multiple flexible loops of various designs, J loops, C loops, closed loops and flexible radial arms.
Later, these loops which became commonly referred to as “haptics” were replaced in some lens designs with flat homogeneous plates, called “plate haptics”. The plate haptic design has two main advantages over loop lenses. First, if they have a plate length of 10.5 to 11.5 mm they vault backwards when confined within the approximate 10.5 mm diameter and 5.0 mm deep space that remains within the human capsular bag after extracting the cataract. Second, their location along the axis of the eye is more consistent than that of loop lenses. These two properties of plate lenses reduce the incidence of the major post-operative complications of cataract surgery, which are retinal detachments and cystoid macular edema, and because of the more consistent location of the lens optic along the axis of the eye, the uncorrected post-operative visual acuities are superior to those of loop lenses.
During constriction of the circular ciliary muscle its diameter decreases and it compresses distal ends of opposing plate haptics which then move centrally. Since the uni-planar plate haptics tend to naturally vault posteriorly when placed within the capsular bag the proximal end of the plates attached to the lens optic move posteriorly. This posterior movement helps to increase the vitreous cavity pressure behind the lens and the lens optic, and pushes the lens optic anteriorly. This increase in vitreous cavity pressure, with a simultaneous decrease in pressure in the anterior chamber of the eye is a natural occurrence in the human eye with accommodation to see at near. However, due to its construction, the lens optic of a traditional plate haptic lens is limited in its response to the change in vitreous pressure.
Furthermore, when a plate haptic lens is placed within the capsular bag of the eye the peripheral circumferential remains of the anterior capsule and the posterior capsule of the human capsular bag, fibrose over the distal ends of the plates. The area of fibrosis can vary and sometimes covers only the distal 1.0 mm of the tip of the plate. With inadequate coverage of the distal ends of the plates the plate haptics can sometimes dislocate, one of the plates vaulting forwards to configure the lens in a “Z” shape configuration.